Help: 'unresolved expression that must be resolved'

[Michael Chapman]

I have trouble trying to refer to a label in the assembler 
anywhere.

The one line test program

	subroutine: call subroutine

gives me

$ gas test1.asm
test1.asm: Assembler messages:
test1.asm:1: Error: unresolved expression that must be resolved
$ 

What does it mean that an expression is unresolved?

Here below is the relevant parts from my .cpu file.

Any explanations as to how to make this simple example work
would be most welcome.

Thanks.
Mike Chapman


; Opcode fields
(dnf f-call-opcode "call opcode" () 7 6)

; Absolute address, 26-bit (shifted 1 bit to right)
(df f-abs26   "abs26"  (ABS-ADDR) 
    31 26 UINT
    ((value pc) (sra WI value (const 1)))
    ((value pc) (sll WI value (const 1))))

; Instruction operands.
(dnop abs26  "abs26"         ()       h-iaddr f-abs26)

; Instructions
(dni call-abs26 "call abs26"
     (UNCOND-CTI)
     "call $abs26"
     (+ (f-call-opcode #xf3) abs26)
     (set pc abs26)
     ()
)